1. Field of the Invention
The present invention relates to an audio processing apparatus which is suitably applied to reproduction of a stereo audio signal of a multi-channels by means of a headphone device.
2. Description of the Related Art
Recently, multi-channel signals are used mostly as audio signals (sound signals) of an image of cinema and the like, and such signals are recorded on the assumption that the signals are reproduced by speakers on both sides and at the center of an image or speakers beyond or on both sides of a listener. As a result, a sound source in the image matches with a position of an actually audible sound image, and a naturally widespread sound field is established.
However, in the case where such a sound is appreciated by using a prior headphone device, a sound image by sound input is localized in a head, and an image position does not match with a sound image localized position so that the sound image localization becomes extremely unnatural. Further, the localization positions of sound signals of respective channels cannot be reproduced separately. Needless to say, a case that only a sound of multi-channels such as musical sounds is appreciated has the similar problems, namely, a sound is heard from a head unlike reproduction via speakers, and thus the sound image localized positions are not separated so that the reproduction of a sound field is extremely unnatural.
As a method which improves this phenomenon, namely, which obtains a sound file which is equal to that in the case of reproducing a sound via speakers even if a sound is heard via a headphone device, there considers a method such that transfer functions from speakers previously positioned for respective channels to both ears of a listener are measured or calculated and is superposed on an audio signal by a filter such as a digital filter, and a sound is heard via a headphone device. FIG. 8 is a diagram showing one example of a prior headphone device to which this method is applied. Stereo audio signals of right and left two channels obtained at input terminals 1L and 1R are converted into digital audio signals by analog/digital converters 2L and 2R. The audio signals of the right and left channels output from the analog/digital converters 2L and 2R are supplied to a digital processing circuit 3. The digital processing circuit 3 is composed of a plurality of digital filters 3LL, 3LR, 3RL and 3RR, and two adders 4L and 4R. The digital processing circuit 3 executes a converting process such that a reproduction sound field similar to a reproduction sound field obtained when a speaker device is positioned indoors is obtained by a headphone device (a process for converting a so-called stereophonic sound into a binaural sound).
A concrete structure of the digital processing circuit 3 is such that an audio signal of the left channel is supplied to the first digital filter 3LL and the second digital filter 3LR, and an audio signal of the right channel is supplied to the third digital filter 3RL and the fourth digital filter 3RR. The respective digital filters have a structure shown in FIG. 9, for example. The digital filter shown in FIG. 9 is an FIR type filter, and a signal obtained at an input terminal 111 is supplied to a plurality of delay circuits 112a, 112b, . . . 112m, 112n which are connected continuously. The signal obtained at the input terminal 111 and output signals of the delay circuits 112a through 112n are supplied to individual coefficient multipliers 113a, 113b, . . . 113n and 113o so as to be multiplied by coefficient values set in the respective multipliers, and the multiplied signals are added respectively in adders 114a, 114b. . . 114m and 114n successively, and an output obtained by adding all the coefficient-multiplied signals is obtained at an output terminal 115.
An output of the first digital filter 3LL having the above structure and an output of the third digital filter 3RL are supplied to the adder 4L so as to be added, and a converted output for the left channel is obtained. Moreover, an output of the second digital filter 3LR and an output of the fourth digital filter 3RR are supplied to the adder 4R so as to be added, and a converted output for the right channel is obtained.
The output of the left channel obtained by addition in the adder 4L is supplied to an digital/analog converter 5L so as to be converted into an analog audio signal. The converted analog audio signal is amplified by an amplifying circuit 6L for driving a headphone, and the amplified signal is supplied to a speaker unit 7L for a left ear in a headphone device 7. Further, the output of the right channel obtained by addition in the adder 4R is supplied to a digital/analog converter 5R so as to be converted into an analog audio signal. The converted analog audio signal is amplified by an amplifying circuit 6R for driving a headphone, and the amplified signal is supplied to a speaker unit 7R for a right ear in the headphone device 7.
There will be explained below a principle that an audio signal for stereophonic reproduction is converted into an audio signal for binaural reproduction in the process in the digital processing circuit 3 with reference to FIG. 10. A speaker device SL for the left channel is positioned on a left front side of a listener and a speaker device SR for the right channel is positioned on a right front side. Audio signals for stereophonic reproduction are reproduced respectively from the respective speaker devices. At this time, as for a sound which reaches a left ear of the listener, a sound arrived from the speaker device SL of the left channel has a transfer function HLL, and a sound arrived from the speaker device SR of the right channel has a transfer function HRL. Moreover, as for a sound which reaches a right ear of the listener, a sound arrived from the speaker device SR of the right channel has a transfer function HRR, and a sound arrived from the speaker device SL of the left channel has a transfer channel HLR.
Coefficient values of the coefficient multipliers of the respective digital filters are set so that the four transfer functions HLL, HLR, HRL and HRR are reproduced according to the operations in the four digital filters 3LL, 3LR, 3RL and 3RR. As a result, two-channel audio signals for stereophonic reproduction are converted into two-channel audio signals for binaural reproduction. In this case, the transfer functions of an impulse response to both ears from the speaker devices of the respective channels are measured in a resonant room, and the coefficient values to be set in the coefficient multipliers of the digital filters are set based on the measured values. FIG. 11 shows one example of the measured impulse response data.
Here, Japanese Patent Publications (Patent No. 2751155 and the like) which have been applied by the inventors of the present invention discloses details of the process for converting the audio signal for stereophonic reproduction into the audio signal for binaural reproduction.
According to the processing apparatus which have been suggested, a sound image is localized outside of a head of a listener. However, more precisely, in the case where an audio signal converted for binaural reproduction is heard by a headphone, transfer functions from right and left speaker units of the headphone to both ears of the listener function, a characteristic slightly different from the case that a sound is heard from an actual sound source appears.
In addition, the transfer functions from the headphone to both ears of the listener vary with types of a headphone. Therefore, in the case where another type of a headphone is used, there occasionally arises a problem that the localizing state of a sound image varies.
The present invention has been achieved in order to solve the above problems, and it is an object of the invention to be capable of realizing accurate sound image localization similar to that in the case of using speakers when an audio signal for stereophonic reproduction is converted into an audio signal for binaural reproduction and a sound is heard via a headphone.